In general, a self-bearing motor is called a bearingless motor or an integrated motor bearing. Since the self-bearing motor does not need to use any mechanical bearings, its volume and weight can be reduced. Further, since the self-bearing motor utilizes a magnetic levitation technology, friction and wear thereof can be minimized. Furthermore, since there is no need for lubrication in the self-bearing motor contrary to the mechanical bearings, effective maintenance and semipermanent use thereof can be made.
FIG. 2 shows a sectional view of an asymmetric conventional 3-phase step motor including a stator 20 with fifteen stator teeth 21 and a rotor 10 with ten rotor teeth 11. Coils (windings) 30 are wound around the stator teeth. As shown in FIG. 2, the conventional step motor is constructed such that the rotor 10 is rotated by magnetic force while coils, which are connected in series and supplied with same phase currents, are excited in alternating order by means of three phase currents supplied from a step motor controller 40. When the coils are excited by the respective phase currents, in case of the step motor shown in FIG. 2, five electromagnets connected in series are simultaneously excited. In the asymmetric step motor, when the coils are excited by the respective phase currents, the electromagnets for generating electromagnetic force are not arranged symmetrically around the circumference of the stator circle.
A self-bearing step motor, which is supported by magnetic force by adding an additional winding without any mechanical bearings, is disclosed in detail in U.S. Pat. No. 4,683,391 entitled “magnetically floating actuator having angular positioning function” issued to Toshiro Higuchi. FIG. 1 is a schematic view of the self-bearing step motor of the '391 patent As shown in FIG. 1, a technique disclosed in the '391 patent is characterized in that a single actuator was allowed to perform the motor and bearing functionalities simultaneously by adding bearing coils (bearing windings) 60 for generating the magnetic force to support a rotor 80 to torque coils (torque windings) 50 of electromagnets of the conventional step motor. Further, the '391 patent discloses that X-axis and Y-axis motions of the rotor 80 are controlled in a state where a stator 70 is divided into four segments.
However, in the technique disclosed in the '391 patent, the structure of the stator 70 should be separately designed to accommodate the windings of the bearing coils, and additional bearing coils and equipments for controlling the additional bearing coils are needed. Thus, the substantial effect of reduction in volume and weight of the step motor owing to elimination of the mechanical bearing is not too great. In particular, there is a problem in that the technique is applicable only to a stator having a symmetric structure.
A self-bearing step motor, which neither uses a mechanical bearing nor includes additional windings for supporting a stator, is disclosed in U.S. Pat. No. 5,424,595 entitled “integrated magnetic bearing/switched reluctance machine” issued to Mark A. Preston et al. The self-bearing step motor disclosed in the '595 patent is characterized in that it comprises a rotor having rotor teeth and a stator having stator teeth wound with windings and the windings are separated from one another and simultaneously excitable. Further, the self-bearing step motor of the '595 patent is characterized in that the same phase currents are distributed among the respective windings in an inversely proportional manner. Furthermore, it is characterized in that the stator teeth are disposed in diametrically opposite pairs.
However, the '595 patent is merely directed to a hetero polar type of step motor. In such a case, it is difficult to perform stable control of the self-bearing step motor, because a magnetic flux direction is changed upon rotation of the rotor. In particular, it is difficult to apply such a self-bearing step motor to a case where the stator teeth are a symmetrically arranged.
The features of the conventional magnetic levitation technology are that the windings are symmetrically disposed to magnetically float an object as shown in FIG. 1. Since the electromagnets are disposed to be symmetric with respect to the X- and Y-axes, four electromagnets are generally needed for performing a magnetic bearing function.
On the other hand, an asymmetric step motor is still widely utilized. The conventional self-bearing technology does not teach or suggest any control algorithms for allowing such an asymmetric step motor to be used as a self-bearing motor in the absence of the bearing coils.
In addition, the step motor causes the excited state of the electromagnets to be changed in regular order so as to rotate the rotor. The sequential change of the driving electromagnets according to such a phase change is one of the difficulties in developing the conventional step motor into the self-bearing step motor. In particular, since a circumferentially overlapped length of the stator and rotor teeth is changed upon rotation of a shaft of the motor, an overlapped sectional area through which the magnetic flux flows is also changed. Thus, change of magnetic force magnitude due to the sectional area change is also one of the causes of obstruction to the development of the self-bearing step motor.